The torsion beam wheel suspension system including a pair of trailing arms rigidly connected to each other by a torsion beam is known for use as a rear wheel suspension system of front wheel drive vehicles. The torsion beam wheel suspension system has the advantage of providing a favorable ride quality and a stable turning performance in spite of the simple and compact structure thereof. In a vehicle fitted with a torsion beam wheel suspension system, the front end of each trailing arm is pivotally connected to the side sill extension on the corresponding side.
An automobile normally carries a spare tire for the contingency of having a flat tire. A spare tire is typically placed below a rear floor or inside a spare tire pan formed by downwardly bulging a floor panel. Also, most of the currently available vehicles use monocoque vehicle bodies which can be relatively light in weight while ensuring a required rigidity. The floor frame of a monocoque vehicle body is formed by combining stamp formed members made of relatively thin steel sheet. Therefore, to ensure an adequate rigidity to the rear vehicle body structure against a rear end collision while forming a space for receiving a spare tire, the thickness of the steel sheet has to be increased in a corresponding manner or other measures are required to be taken.
However, it is desired that the thickness of the steel sheet be not increased because an increased thickness means an increase in the weight of the vehicle which in turn impairs the fuel economy and reduces the dynamic performance of the vehicle. To the end of improving the absorption of the impact energy at the time of a rear end collision and protecting a fuel tank at the time of a rear end collision in a reliable manner, it has been proposed to join a pair of rear side frames with a middle cross member and a rear end member at front and rear ends of the rear side frames, respectively, and extend a rear center frame having an upwardly slanted upper surface toward the front part thereof across the middle cross member and rear end member on the lower surface of the rear floor panel so that a spare tire may be received in such a manner that the spare tire abuts the lower surface of the rear center frame. See Japanese patent laid open publication No. 2002-362426, for instance. In this arrangement, the spare tire and rear center frame can be used as members for absorbing energy and the spare tire is prevented from moving forward at the time of a rear end collision.
However, according to the invention disclosed in Japanese patent laid open publication No. 2002-362426, the lower surface of the rear center frame is slanted upward toward the front so that the front part of the rear center frame is made prone to buckling. At the time of a rear end collision, once the rear center frame has buckled, the rear center frame is unable to absorb an adequate amount of energy. Also, because the spare tire is placed under the rear center frame or under the rear floor panel, taking out and putting back the tire from and into the designated space when changing the tires cannot be easily performed, and the user may not be able to change tires without soiling his clothing. Furthermore, the spare tire is retained in an exposed state under the rear floor panel so that the spare tire gets soiled very easily, and it could create some difficulty in inspecting the spare tire. Therefore, there is a demand for a vehicle body structure which is relatively safe against a rear end collision and simplifies the inspection of the spare tire and the work involved in changing tires.